It can depend on the person, but now I let them know beforehand especially if there is anything I really don't want.
What get to me is when people know I like classical music (or what ever it might be) and I say I have loads of Me to You bears and people should perhaps not buy me one of those (if they get me anything at all) as I most likely have got it already, (as I have so many how are they to know which I have or not) when it came to giving out presents a few years back, I got a Me To You bear, one I already had, everyone else got other really nice stuff, two people even got nice classical box sets with loads of different artists in. I said ooh they look nice, the person who gave me a gift said if I had known you liked classical music I might have got you one too. When I tell someone one of my favourite songs is Beethoven's 6th symphony, I think thats a big clue.
But you know what, really its the thought that counts and we shouldn't look a gift horse in the mouth, yes I got a bear I already had, but so what, I actually like having things to open regardless of the content (obviously there are exceptions)..
I have made a point of letting everyone know NO Me to You bears, and when it comes to my hubby (again if they are nice enough to get him something) I usually (rather then hinting) just cut to the chase and say anything like an old airplane or space related is fine..
I think you do have to tell some people, other wise how do they know, and its not fair on them. They spend money hoping to get you something good, and its not, and then you end up being disappointed, for what?